In a case when a digital signal is transmitted via a transmission line, due to a portion of the signal energy being reflected when the signal level changes, distortions in the waveform such as overshoots and undershoots, that is, ringing, occur on the receiving side. Various proposals are made to suppress waveform distortion, for example, in the following patent documents.
[Patent Document 1] JP 2001-127805A (U.S. Pat. No. 6,326,803B1)
[Patent Document 2] JP 2010-103944A
For example, in patent document 1, in a case when the voltage level of the signal transitions between high and low in a terminal circuit of a transmission path, the impedance of a terminal is temporarily decreased during a delay time that is conferred by a delay circuit. Further, an auxiliary switching circuit is connected in parallel to a terminal switching circuit that is used in the prior art. Four MOSFETs are connected to the auxiliary switching circuit in series between a power source Vcc and a ground, and the switching control thereof is performed by a signal that is transmitted to the terminal and a signal in which such a signal is delayed and inverted by inverters in a series of three. However, with such a configuration, when the terminal is temporarily connected to the power source Vcc or the ground, the on-resistance of the plurality of MOSFETs is transiently connected between both parts in series or in both series and parallel. It is therefore not possible to sufficiently decrease the impedance of the terminal. While it is necessary to increase the size of the FETs in order to decrease the on-resistance, the size of the terminal circuit would inevitably increase.
Further, in patent document 2, a switch is connected between a high voltage signal line and a low voltage signal line that transmit a differential signal. When a waveform distortion detection unit detects that the relative size of the voltage between the lines and has been reversed, the switch is closed and short-circuiting is caused between the lines and. If, short-circuiting is caused between the lines and the impedance between the lines becomes zero, the distortion in the signal waveform in the vicinity of the node that receives the transmitted signal can be reduced. However, since the energy of the distortion components of the waveform is not consumed in the case of a short circuit, the energy is reflected at the short circuit point and reaches the side of the node that has transmitted the signal. This is adversary to other nodes.